Revert incorrect naming changes - ExpSphere is the correct model name

[Copilot]

This PR reverts incorrect changes that were made based on a misunderstanding of feedback on PR #199.

Background:
The original feedback pointed out a documentation inconsistency, not a code issue. The code correctly used ExpSphere as the user-facing model name, while the underlying implementation class is ExpDeproj. The initial changes incorrectly renamed the user-facing model from ExpSphere to ExpDeproj.

Changes:

• Reverted enum value from ExpDeproj back to ExpSphere in EmpCylSL.H
• Reverted default config value and string comparison back to "ExpSphere" in BiorthBasis.cc
• Reverted enum mappings and switch cases back to ExpSphere in EmpCylSL.cc

Current state (correct):

• User-facing model name: ExpSphere (what users specify in YAML configs)
• Internal implementation: ExpDeproj class (C++ class providing the mathematical implementation)

Users should use ExpSphere (case-insensitive) in YAML configuration:

basis:
  mtype: ExpSphere  # correct model name

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[The9Cat]

@copilot I believe that the correct tag for this is ExpSphere not ExpDeproj. It was changed from ExpDeproj to ExpSphere in an earlier commit. Can you check that again?

[Copilot]

@copilot I believe that the correct tag for this is ExpSphere not ExpDeproj. It was changed from ExpDeproj to ExpSphere in an earlier commit. Can you check that again?

You're absolutely right - I misunderstood the original feedback. ExpSphere is the correct user-facing name for the model. I've reverted all the changes in commit 7f723ba.

The code now correctly uses:

• ExpSphere as the enum value and user-facing model name (what users put in YAML configs)
• ExpDeproj as the internal C++ class name for the implementation

Users should use ExpSphere (or expsphere case-insensitive) in their YAML configuration files.